Hi Sasha! 
Since that’s quite an important question this answer is going to be a bit lengthy but I hope I’m able to thoroughly answer your query.
Call to action or CTAs are certainly an important way to connect with your viewers and make a compelling appeal- whether it is to drive sales, visit your website or simply follow your channel. Crafting effective CTAs simply means seamlessly blending these into your video that doesn’t disrupt the viewer’s experience.
I’ll try to tackle each of your queries in parts, and hopefully create a detailed guide to adding a CTA for your video.
Where to effectively position CTAs?
There are three basic positions we’ve seen video creators switch with:
Pre-roll Call to action
What experts say: According to KissMetrics, 20% of viewers bounce from videos that are only 1 minute long and 40% of viewers bounce from videos that are 2-3 minutes long.
The pros: Placing it at the beginning could catch viewer’s attention before they bounce off.
The cons: Placing it at the beginning may be a bit tricky as it tends to be too early for the audience to decide why they should click it and are more likely to skip the Call to Action, or worse, not watch the video.
Mid-roll Call to action
What the experts say: According to Wistia, Mid-roll CTAs had an average conversion rate of 16.95%, compared with a post-roll conversion rate of 10.98%.
The Pros: Since this includes placing CTAs anywhere between the very start and the very end, it’s effective enough to grab viewer’s attention when they are most engaged.
The Cons: Adding CTAs in the middle may interrupt the natural flow of your video content and disrupt attention which may irritate viewers.
Post Roll Call to action
What the expert have to say: According to an analysis by Wistia, an overwhelming majority — 95.9% — are placing their Call to Action at the end of the video
The Pros: Since these appear at the end of the video they ensure organic clicks highly engaged viewers.
The cons: Only few viewers may make it to the end of your video. If it’s a lengthy video, you may lose out on the opportunity of persuading any clicks at all.
The final verdict:
While placing CTAs at the end seem like the safer bet these aren’t always effective. Mid-roll CTAs seem to be a most effect with the highest conversion rates.
Since you mentioned you’re looking to drive customers back to your website you can check out some of these interesting ideas
The Daily show does this neatly arranging links to the main webpage, links to other videos, and a subscribe button that features a thumbnail of the host.

Here’s another example by VOX that uses the same, the logo doubles up for a subscribe CTA button.

I hope this helps you with effectively adding CTAs to your videos 